We must define the macro "printf" with arguments, else it can mess
up format archetype attributes in builds where PG_PRINTF_ATTRIBUTE
is just "printf".  Fortunately, that's easy to do now that we're
requiring C99; we can use __VA_ARGS__.

On the other hand, it's better not to use __VA_ARGS__ for the rest
of the *printf crew, so that one can take the addresses of those
functions without surprises.

I'd proposed doing this some time ago, but forgot to make it happen;
buildfarm failures subsequent to 96bf88d5 reminded me.

Discussion: https://postgr.es/m/22709.1535135640@sss.pgh.pa.us
Discussion: https://postgr.es/m/20180926190934.ea4xvzhkayuw7gkx@alap3.anarazel.de